DESCRIPTION (Verbatim from Applicant's Abstract): Yeast/mold dimorphism in Histoplasma capsulatum (Hc) is critical to the pathogenesis of the lung disease histoplasmosis which infects an estimated 500,000 Americans each year. The fungus grows in a differentiated multicellular mold form in soil but converts to a undifferentiated single-cell yeast form after inhalation of spores or mold fragments. Studies have shown that Hc treated to block this mold-to-yeast (M-Y) shift is incapable of causing disease. Thus, an understanding of the molecular basis of dimorphism is important for diagnosis and treatment. In this project we propose to study the molecular biology of the Y-M and M-Y shift by isolating and analyzing structural and regulatory genes involved in dimorphism. Genes representing both abundant and rare transcripts strongly up-regulated in each morphologic form will be isolated from non-normalized and normalized subtracted libraries. Two candidate clones already isolated will be studied in detail to determine their possible role in dimorphism. This work will proceed with the following three aims: 1 - Isolate dimorphism regulated genes from subtracted libraries. 2 - Isolate and analyze the complete gene of a mold-specific clone (MS8) and a yeast up-regulated clone (Y86) which is homologous to thiol specific antioxidant protein genes. 3 - Determine the effects of MS8 and Y86 gene knockouts on cell growth and dimorphism. This work will be a strong base for more in depth research in the future and yield important data to increase our understanding of dimorphism and its relationship to virulence in this important pathogen of man.